The present invention relates to a digital mobile communication system and transmission/reception method thereof, and more particularly to a digital mobile communication system and transmission/reception method thereof in which a transmission station and receiving station transmit or receive signals multiplied by an arbitrary time window.
Transmission methods such as OFDM (Orthogonal Frequency Division Multiplex) that use FFT (Fast Fourier Transform) and CP (Cyclic Prefix) have been examined as radio access methods for next generation mobile communication. It is well know that the OFDM method has robustness to frequency selective broadband radio channels.
The theory of the OFDM method will be explained below. FIG. 19 is a block diagram of a typical transmission station in the OFDM method. First, an error correction encoding unit 1 performs encoding of a data signal so that a receiver can execute error correction, a data modulation unit 2 performs data modulation, and a data/pilot signal multiplexing unit 3 performs time-division multiplexing of a data signal and a pilot signal that is known by the receiving station. Next, an IFFT unit 4 performs IFFT processing in unit of a set number of samples, and a CP insertion unit inserts a CP. The aforementioned set number of samples is called an OFDM symbol.
More specifically, as shown in FIG. 20, by copying NCP number of samples at the end of each of the OFDM symbols (=NFFT samples) after IFFT and inserting them at the start of each of the OFDM symbols, the CP serves the role as a guard interval for each of the OFDM symbols. Here, the CP is cyclically copied, so the signal continues for the interval of (NFFT+NCP) after the insertion of the CP. Next, a DA conversion unit 6 performs D/A conversion and a transmission RF unit performs orthogonal modulation, converts the baseband signal to a radio frequency signal and transmits that radio signal toward a receiving station 9 from a transmission antenna 8.
FIG. 21 is a block diagram of a typical receiving station in the OFDM method. A reception RF unit 10 converts the signal that is transmitted from the transmission station from a radio frequency signal to a baseband signal and performs orthogonal demodulation, after which an AD conversion unit 11 performs A/D conversion. A FFT timing detection unit 12 detects the reception timing of a direct wave by calculating the correlation between the received signal and a replica of the transmission pilot signal. A CP removal unit 13 removes the CP from the received signal based on the reception timing information, and extracts the effective signal component from each OFDM symbol.
FIG. 22 is an example showing the state of extracting the effective signal component. For convenience of explanation, the received signal is divided into and expressed as path components (direct wave, indirect wave), and ignoring the effects of thermal noise. It can be seen that only the effective signal component of OFDM symbol n after the CP has been removed is accurately extracted from the direct wave of path 1 and a signal component having part of the CP is extracted from the indirect wave (delay wave) of path 2. However, the CP is cyclically copied from the effective signal component of the OFDM symbol, so as a result only the effective signal component of OFDM symbol n is accurately extracted. In other words, multipath components having a delay time that is equal to or less than the CP length are received without interference occurring between OFDM signals.
A FFT unit 14 performs FFT on the signal after the CPs have been removed, and a data/pilot signal separation unit 15 separates the time-division multiplexed data signal and pilot signal from the received signal. A channel estimation unit 16 calculates the correlation between the received pilot signal and a replica of the transmitted pilot signal and estimates the channel distortion in the radio channel. On the other hand, a channel compensation unit 17 multiplies the received data signal by the complex conjugate of the channel estimation value to suppress channel distortion, a data demodulation unit 18 uses the compensated received signal to demodulate the received data, and an error correction decoding unit 19 performs error correction and decoding of the demodulated data.
Next, the frequency spectrum of a transmission signal in the OFDM method will be considered. FIG. 23 is an example of the frequency spectrum of the signal that is transmitted from the transmission station shown in FIG. 19. The horizontal axis is the frequency that has been normalized by the system bandwidth, and the vertical axis is the power that has been normalized by the transmission power at near the center frequency. In this example, the power on the outside of the effective subcarrier of the system bandwidth gradually converges, so the radiation at the adjacent band is large. This is due to the frame format of the OFDM method shown in FIG. 20. As was described above, in the signal after the CP is inserted, the signal continues inside one OFDM symbol (=NFFT+NCP samples), however, the signal does not continue at the boundaries of each OFDM symbol. This means the signal is multiplied by a rectangular window function having the unit of an OFDM symbol, and thus the frequency spectrum has a waveform in which the Sinc function is convoluted, and the power converges gradually.
Multiplying the signal by a window function other than a rectangular window function so that the signal is gradually damped at the boundaries of the OFDM symbols is known as a method for reducing the adjacent band radiation. FIG. 24 shows the state of multiplying the signal after CP insertion (OFDM symbol A) by a window function. First, taking the interval during which the signal is damped by the window function to be Nwin samples, then Nwin/2 samples that are cyclically copied at two locations of each OFDM symbol are inserted on both sides of the OFDM symbol A (see (A) and (B) of FIG. 24). The signal continues during the interval of (NFFT+NCP+Nwin) samples after insertion. Next, each of the intervals of Nwin samples on both sides of this interval of (NFFT+NCP+Nwin) samples is multiplied by a window function (see (C) of FIG. 24). Here, a raised cosine function is used as the window function. After that, the OFDM symbols are joined so that the intervals in which the signal is damped by the window function overlap each other between adjacent OFDM symbols (see (D) of FIG. 24). FIG. 25 shows the frequency spectrum of the transmission signal when multiplied by the time window of the raised cosine function. The signal is damped near the non-continuous points at the boundaries of the OFDM symbols, so the power converges more quickly than when the signal is multiplied by a rectangular time window.
In addition, it is also possible to reduce the adjacent band radiation by using a band pass filter having a precipitous frequency characteristic.
FIG. 26 shows the state in which the CP removal unit 13 of the receiving station extracts the effective signal component of each OFDM symbol from the received signal in the case where the signal is multiplied by the window function described above by the transmission station. As in the case shown in FIG. 22, when extracting signals using a typical extraction interval ITI in the OFDM method, only the effective signal component of the OFDM symbol n is accurately extracted from the indirect wave of path 2. However, in the direct wave of path 1, the effective signal component of the OFDM symbol n is distorted at the tail end Ttail of the extraction interval, and furthermore, a signal from the adjacent OFDM symbol (n+1) is mixed in as interference, so the final reception deteriorates.
In order to avoid this problem, the receiving station must extract the received signal so that it does not include the time window area. When the overall communication system is regulated so that common window function processing is performed by each transmission station, it is possible to easily decide the position for extracting the received signal by the receiving station by taking into consideration that common window function processing. However, in a case where just the upper limit value of the adjacent band radiation is regulated and the system is not regulated such that common window processing is performed, the designer appropriately selects a method for reducing the adjacent band radiation when designing the transmission station according to restrictions on the supported transmission speed and circuit scale. In this state, even when each of the transmission stations performs window function processing, it is feasible that the method for applying the time window, for example using a different window width, will differ. In that case, it is not possible to easily decide the position for extracting the received signal by the receiving station.
One method for solving this problem, regardless of whether or not window function processing is actually performed on the transmission signal, is to always shift the extraction interval forward by the receiving station as shown by the cut-out interval IT2 in FIG. 26 so that the time window area is not included in the extraction interval. However, when the extraction interval is shifted forward, an adverse effect occurs in that the substantial CP length becomes shorter, so even when the delay time of the signal in the delay path is short, inter-symbol interference occurs. Therefore, when the extraction interval is shifted forward for a transmission signal to which a time window has not actually been applied, the CP is not effectively used, so it cannot be said that the method of simply shifting the extraction interval forward is the best method.
A technique has been proposed in which in order to suppress discontinuity between symbols of the OFDM modulated signal, the amount of computation of window function processing is reduced (Refer to JP2003-3480421 A). In addition, an interference removal device has been proposed that determines whether or not the delay difference between a certain user and another user is equal to or greater than the window width of the time window, and removes the interference component (Refer to JP2003-347947 A). However, neither of these techniques extracts the effective symbol component from the received signal and performs demodulation so that the signal does not include the time window region.